Apparatus and method for automatic differential pressure transducer range changing

ABSTRACT

In one exemplar embodiment, apparatus for automatic differential pressure transducer range changing is disclosed, utilizing a single differential pressure transducer the differential pressure measurement range of which may be changed merely by changing the characteristics of the electrical circuit generating a signal representative of the measured differential pressure and a signal level detector circuit to receive the output signal from the transducer and generate a signal the polarity of which changes in response to a change in level of the transducer output signal beyond a predetermined limit representative of a change in measurement range of the transducer. The Polarity-changing output signal of the detector circuit actuates a switching circuit to switch appropriate predetermined resistive elements into the transducer amplifier gain circuit to incrementally change the amplifier gain, and hence, the measurement range of the transducer.

United States Patent [1 1 'Stroman [m A 3,713,337 51 Jan. 30, 1973 [s41APPARAT S AND METHOD FOR AUTOMATIC DIFFERENTIAL PRESSURE TRANSDUCERRANGE [58] Field of Search ..73/407, 205, 397, 35, 398; 324/115; 323/66[56] 5 References Cited UNITED STATES PATENTS 3,514,209 5/1970 McGhee etal. ..324/l15 X 3,555,901 1/1971 Delatorre et al. 73/205 R X 3,350,94511/1967 Bristol ..73/407 R I M DIFFERENT/AL PRESSURE T TRANSDUCERPrimary Examiner-Charles A. Ruehl [Atrorney-Arnold, White & Durkeeand,Darryl M.

Springs [57] ABSTRACT In one exemplar embodiment, apparatus forautomatic differential pressure transducer range changing is disclosed,utilizing a single differential pressure transducer the differentialpressure measurement range of which may be changed merely by changingthe characteristics of the electrical circuit generatinga signalrepresentative of the measured differential pressure and a signal leveldetector circuit to receive the output signal from the transducer andgenerate a signal the polarity of which changes in response to a changein level of the transducer output signal beyond a predetermined limitrepresentative of a change in measurement rangeof the transducer. ThePolaritychanging output signal of the detector circuit actuates aswitching circuit to switch appropriate predetermined resistive elementsinto the transducer amplifier gain circuit to incrementally change theamplifier gain, and hence, the measurement range of the transducer.

7 Claims, 3 Drawing Figures -FLOW COMPUTER LEVEL DETECTOR 24 ourPur' v VOUTPUT PATENIEnmaoms 3.713.337,

FIG. I

DIFFERENTIAL Y FLOW 4 PRESSURE E 1 TRANSDUCER COMPUTER L 22 r [E 16LEVEL DETECTOR 24 30 25 35 RANGE 25\ lNFO To ZERO INPUT GAIN INPUT LEVELDETECTOR 46 FIG 3 V 4 .(m I

TRANSDUCE 7 Larry J. Stromqn INVENTOR 4. (m V) 20 E BY 0 20 100 200Amod, W/uiel: Uwnfiee E DIFFERENT/AL PRESSURE (INCHES H2 0) ATTORNEYSAPPARATUS AND METHOD FOR AUTOMATIC DIFFERENTIAL PRESSURE TRANSDUCERRANGE CHANGING BACKGROUND OF THE INVENTION A common problem in orificemeasurement of flowing fluids is the rangeability of the differentialpressure measuring apparatus. This problem results largely from the factthat the orifice flow measurement is calculated by extracting the squareroot of the prime variable, differential pressure. If a measurementrange of :1 is utilized in the differential pressure transducermeasuring the differential pressure across the orifice, resulting flowmeasurementrange is only slightly greater than 3:1. Often the requiredflow measurement range exceeds this value and it becomes necessary toprovide other means for handling an extended range of differentialpressure measurement.

Extended ranging of differential pressure measurement is commonly doneby either of two methods. The first method is to add additional meterruns in parallel, however, this is exceedingly expensive, since itduplicates all of the equipment in each parallel meter run. The secondmethod is to add an additional transducer or transducers, the range ofeach of which is sized such that it extends the overall measurementrange. A common measurement system for extending the flow range is toutilize two transducers, for example, 0 to and 0 to 200 inches H,O. Withthis arrangement, the 20-inch transducer is used when it is in range andthe 200-inch transducer is used when the 20-inch transducer is out ofits range. Differential pressure can then be measured from 2 to 200inches and never go beyond a 10:1 range on a given transducer.Differential pressure measured from 2 to 200 inches H,O is adifferential pressure variation of 100:1 and yields a flow measurementvariation of 1021. This is an acceptable extension over the 3:1 examplehereinabove mentioned. This technique is in common use. However, it hasa basic limitation in that two differential pressure transducers must beprovided which adds to the expense and complication of the system.

The present invention remedies the problems of the prior art byproviding method and apparatus for utilizing a single differentialpressure transducer whose differential pressure measurement range may bechanged merely by changing the characteristics of the electrical circuitgenerating a signal representative of the differential pressure measuredby the device. One such transducer is the Series 400 transducersmanufactured by KDG Instruments, Ltd. of Great Britain. The disclosedapparatus and method allow for utilization of a single transducer andautomatic changing of the differential pressure measurement range asrequired to achieve the same performance as may be obtained by utilizingtwo separate transducers.

SUMMARY OF THE INVENTION The present invention provides novel apparatusand method for automatically measuring a plurality of ranges ofdifferential pressure of a fluid flowing across a pressure headproducing device, i.e., an orifice plate, within a pipeline and utilizesa transducer means for measuring differential pressure across theorifice plate and an amplifying means cooperating with the transducermeans to generate a first electrical signal representative of themeasured differential pressure within a first measurement range. Asignal level detector means receives the first electrical signal,detects the level of the electrical signal and generates a secondelectrical signal, the polarity of which changes in response to a changein level of the first electrical signal exceeding a predetennined signallevel limit representative of a change in measurement range of thetransducer means. A variable gain means cooperates with the amplifiermeans for automatically changing the gain of the amplifier means foraccomplishing a change in measurement range of the transducer means. Thevariable gain means is actuated by a switching means responsive to thesecond electrical signal generated by the signal level detector.

Accordingly, one primary feature of the present invention is to providean automatic differential pressure range changing circuit that utilizesonly a single differential pressure transducer the differential pressuremeasurement range of which may be changed merely by changing thecharacteristics of the electrical circuitry that generates an electricalsignal representative of the measured differential pressure, therebyeffecting a substantial cost saving.

Another feature of the present invention is to provide a means ofautomatically changing the differential pressure measurement range of anelectrical differential pressure transducer means and thereby expandrangeability of the orifice measuring system.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the manner in which theabove-recited advantages and features of the invention are attained, aswell as others which will become apparent, can be understood in detail,a more particular description of the invention may be had by referenceto specific embodiments thereof which are illustrated in the ap' pendeddrawings, which drawings form a part of this specification. It is to benoted, however, that the appended drawings illustrate only typicalembodiments of the invention and therefore are not to be consideredlimiting of its scope for the invention may admit to further equallyeffective embodiments.

In the drawings:

FIG. 1 is a schematic block diagram of the automatic differentialpressure range changing circuit herein disclosed.

FIG. 2 is a schematic diagram of the level detector circuit, theswitching circuit and the variable gain circuit utilized in theautomatic differential pressure range changing circuit herein disclosed.

FIG. 3 is a graphical representation of the detector circuit output asit correlates to the output signal of the differential pressuretransducer for a plurality of differential pressure ranges in accordancewith the invention herein disclosed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG.I, a pipe 10 is shown in crosssection having a fluid such as gas flowingtherein in the direction shown by the arrow. For fluid flow measurementpurposes, an orifice plate 12 is shown inserted in pipe 10, the orificeplate 12 having an orifice or aperture 13 for allowing the passage ofthe fluid but creating a pressure head across the orifice plate 12 forproducing a differential pressure between the upstream and downstreamsides of the plate. The differential pressure measurement is commonlyutilized in calculating the flow rate and volume of the fluid movingthrough the pipeline 10. An electrical differential pressure transducermeans 14, the differential pressure measuring range of which may bechanged by merely changing the characteristics of the electrical circuitthat generates an output signal representative of the measureddifferential pressure, is utilized to measure the differential pressureacross orifice plate 12.

Electrical differential pressure transducer means 14 includes adifferential pressure transducer means 16 which measures thedifferential pressure across the pressure producing head or orificeplate 12 and produces electrical signals representative of the upstreamand downstream pressures across the orifice plate which are applied viaconductors 19 and 21 to an amplifier means 18. The amplifier means 18 istypically a differential amplifier which subtracts the signalsrepresentative of the upstream and downstream pressures across theorifice plate 12 and generates an electrical signal representative ofthe differential pressure across the orifice plate. As hereinabovementioned, the Series 400 transducer manufactured by KDG Industries,Ltd. may be advantageously utilized for transducer means 14.

This differential pressure signal is applied via conductor 20 to a flowcomputer 26 for utilization in calculating the flow rate and volume ofthe fluid moving through pipe 10. The electrical signal representativeof the differential pressure is also applied via conductors 20 and 22 toa level detector and switching circuit 24 where the signal level of theelectrical signal representative of the differential pressure ismonitored. A level detector means or circuit senses a change in level ofthe electrical signal representative of differential pressure exceedinga predetermined signal level limit, representing a change in themeasurement range of the differential pressure transducer means. Thedetector means actuates switching means within the level detector andswitching circuit 24 to actuate a variable gain means interconnected inthe gain control circuit of amplifier means 18 to change the gain of theamplifier by a predetermined value and accomplish a change in thedifferential pressure transducer measuring range. A signal representingthe output signal of the detector circuit is also applied via conductor25 to the flow computer 26 for reasons to be hereinafter furtherexplained.

The variable gain means may 'be accomplished as shown in FIG. 1 byutilizing variable resistors 32 and 33, one side of each of which isconnected to a common source potential in the gain circuitry ofamplifier means 18, and the other side of each of which is connected torelay contacts 28 so that either resistor 32 or 33 may be inserted intothe gain control circuit of amplifier means 18 for varying the gain ofthe amplifier by a predetermined value. Similarly, when the differentialpressure measurement range of the electrical differential pressuretransducer means 14 is changed, its zero point will also change and,accordingly, a'zeroing means to adjust the zero point when the gain ofamplifier means is changed may be included by utilizing relay contact30, ganged to relay contact 28. Variable resistors 34 and 35 areconnected to a common source potential in the zeroing circuitry ofamplifier means 18, and may each be preset to accomplish the appropriatezeroing function when relay contact 30 is appropriately actuated by thelevel detector and switching circuit 24.

Commonly, a measurement system utilizing an extended flow range would belikely to utilize two measurement ranges, for example, 0 to 20 and 20 to200 inches H,O differential pressure. Utilizing these two ranges,differential pressure can be measured from 2 to 200 inches H 0differential pressure, thereby providing a desirable differentialpressure variation of 100:1. Accordingly, two differential pressuremeasuring ranges would be needed and two predetermined amplifier gainand zeroing values would be utilized in the gain and zeroing circuitsassociated with amplifier means 18 above described in order toelectrically accomplish the change in differential pressure measurementrange of the differential pressure transducer means 14.

In FIG. 2, the level detector and switching circuit 24 is shown ingreater detail. The electrical signal representative of differentialpressure is applied via conductor 22 as an input to a buffer amplifier38 that provides a high input impedance and avoids loading resistor 36.The output of buffer amplifier 38 is applied via resistors 42 and 44 andconductor 45 as one input to differential amplifier 46. Differentialamplifier 46 functions as a level detecting circuit. A predeterminedpositive voltage is applied via conductors 74 and 72 and variableresistor as a second input to level detector circuit 46. The outputsignal of level detector circuit 46 is applied via conductor 47 andresistor 48 to the base lead of NPN transistor 49 connected in a commonemitter configuration. The collector lead 50 of transistor 49 isconnected to a relay coil 52 which in turn actuates relay contacts 54.When relay contacts 54 are closed, positive voltage is applied viacontact 54 and conductor 55 to relay coil 56. Energization of relay coil56 actuates relay contacts 28 and 30 and switches either gainresistors'32 or 33 and zeroing resistors 34 and 35 into the circuit forvarying the gain and zeroing functions of amplifier means 18 ashereinabove described.

The output of level detector circuit 46 is also applied via conductors47, S7 and 59 and resistor 58 to the base lead of transistor 60, an NPNtransistor connected in a common emitter configuration. The collectorlead 62 of transistor 60 is connected to a source of positive potentialvia resistor 63 and is also connected via resistor 64 to the base leadof transistor 66, also an NPN transistor connected in a common emitterconfiguration. The collector lead 67 of transistor 66 is connected toresistor68 and a variable resistor 70 and to conductor 72, the secondinput to the detector circuit 46. In addition, the output of leveldetector circuit 46 is applied via conductors 47 and 25 to flow computer26 for reasons to be hereinafter discussed.

. Referring now to FIGS. 1, 2 and 3, and assuming that the electricaldifferential pressure transducer means 14 utilizes two differentialpressure measurement ranges, for example 0 to 20 and 20 to 200 inchesI'I,O, and assuming-that the differential pressure measured is in thefirst or low -measurement range, the first electrical signal generatedby amplifier means 18, representing differential pressure, will beapplied via conductors and 22 as an input to buffer amplifier 38. Theoutput of buffer amplifier 38 is applied via conductor 45 as an input tothe level detector circuit 46. The diflerential pressure signal appliedvia conductor 45 is graphically illustrated in FIG. 3 at 80 in the lowrange, and 82 in the high range. A predetermined positive voltage isapplied via conductors 74 and 72 and resistor 70 as a second input tothe detector circuit 46. The predetermined voltage level applied in todetector 46 via conductor 72 sets the detector threshold level thatseparates the two measurement ranges of transducer means 14. In FIG. 3,the threshold voltage may be approximately 100 millivolts, for example.As long as the signal applied via conductor 45 to the input of leveldetector circuit 46 is less than the predetermined threshold potentialvia conductor 72, the output of detector circuit 46 will be a positiveelectrical signal. The detector output signal is graphically shown at 76in FIG. 3. This positive electrical signal when applied via conductor 47and resistor 48 to the base lead of transistor 49 causes the transistorto conduct and energize relay coil 52,-thereby closing relay contact 54.When relay contact 54 is closed, voltage is applied from a positivevoltage source via contact 54 and conductor 55 to energize relay coil56, thereby actuating relay contacts 28 and 30 to switch gain resistor33- and zeroing resistor 35 into the circuit for the amplifying means18. The electrical differential transducer means 14 is now set in itslow differential pressure measurement range. In the low range, thedifferential pressure signal applied in a conductor 45 to detector 46 isillustrated in FIG. 3 at 80.

With electrical differential pressure transducer 14 in its low range,i.e., the output of detector 46 is a positive electrical-signal, thesignal applied via conductors 47, 57 and 59 and resistor 58 to the baselead of transistor 60. With a positive potential applied to its baselead, transistor 60 will conduct, applying a negative bias to the baselead of transistor 66 via emitter lead 62 and resistor 64, assuring thattransistor 66 is biased to cut-off and will have no effect on thevoltage potential applied to detector circuit via conductor 72 as thedetector threshold level.

When the signal applied via conductor 45 as an input to level detectorcircuit 46 exceeds the predetermined positive voltage potential appliedvia conductor 72, i.e.,

100 millivolts, for example (FIG. 3), the output signal of detectorcircuit 46 changes polarity and becomes a negative signal showngraphically in FIG. 3 at 78. The negative signal output of detector 46applied to the base lead of transistor 49 biases transistor 49 tocut-off and deenergizes relay coil 52, thereby opening relay contact 54.The opening of relay contact 54 deenergizes relay coil 56 and actuatesrelay contacts 28 and 30 to switch gain resistor 32 and zeroing resistor34 into the gain and zeroing circuits for amplifying means 18. Thisswitching action switches the gain of amplifying means 18 to its secondor high differential pressure measuring range until such time as theelectrical signal applied via conductor 45 as an input to detectorcircuit 46 drops below the value of the adjusted predetermined positivepotential applied via conductor 72 as 6 When the output signal from thelevel detector circuit 46 changes to a negative potential, as when thedifferential pressure measuring range moves from its low to its highmeasuring range, the negative signal applied via conductors 47,57 and 59and resistor 58 to the base lead of transistor 60 biases transistor 60to cut-off and allows a positive potential to be applied via conductor62 and resistors 63 and 64 to the base lead of transistor 66,causingtransistor 66 to conduct. With transistor 66 conducting, the potentialapplied via conductor 72 as an input to the level detector circuit 46will drop to a lower value, approximately 10 millivolts, for example, asshown at 83 in FIG. 3. Resistors 68 and 70 are so sized that the lowervoltage applied via conductor 72 when transistor 66 is conducting willbe the low-level switching point where the transducer means 14 willrevert to low range from the high range.

As may be seen in FIG. 3, when the output of detector circuit 46 ispositive, the differential pressure transducer means 14 output remainsin its low range (i.e., 020 inches H O, for example) as represented bythe graphical representation at 80, and when the detector circuit 46output changes polarity to a negative potential as graphically shown at78, the differential pressure transducer means 14 output shifts to ahigh range (i.e., 20-200 inches H O, for example) as representedgraphically at 82.

Transistors 60 and 66 provide a hysterisis circuit for lowering thelevel detector 46 threshold level as applied in via conductor 72 to thelevel shown graphically at point 83 in FIG. 3, where the high levelrange reverts back to the low level measuring range. When thedifferential pressure signal applied via conductor 45 falls below thelevel shown at point 83 in FIG. 3, the output of detector 46 becomespositive, as shown at 77 in FIG. 3, thereby switching the gain ashereinbefore described and causing transistor 60 to conduct and biasingtransistor 66 to cut-off. The threshold level applied via conductor 72to detector 46 again rises to the level indicated at point 86 in FIG. 3and changes in the differential pressure signal cause the detectorcircuit 46 to respond as hereinbefore described. In shifting from highrange to low range, the differential pressure signal is graphicallyshown at 82, 84, 85 and then tracks the line at 80.

The output of detector 46 is also applied via conductors 47 and 25 toflow computer 26. The detector output signal is applied to flow computer26 to supply conventional range information to the flow computer forchanging scaling factors in the computer for calculation of flow rateand volume.

Numerous variations and modifications may be obviously made in thestructure herein described without departing from the present invention.Accordingly, it should be clearly understood that the forms of theinvention herein described and shown in the figures of the accompanyingdrawings are illustrative only and are not intended to limit the scopeof the-invention.

What is claimed is:

1. An apparatus for automatically measuring a plurality of ranges ofdifferential pressure of a fluid flowing across a pressure headproducing device within a pipeline, comprising a difi'erential pressuretransducer for measuring the differential pressure across the pressurehead producing device,

amplifier means cooperating with said transducer for generating a firstelectrical signal representative of said measured difi'erential pressurewithin a first measurement range,

predetermined resistance for changing the electrical zero of saidamplifier means when different ones of said variable resistors isconnected with the amplifier means circuit.

signal level detector means for comparing the signal The apparatus asclaimed in claim 4, wherein said level of said first electrical signalagainst an ele ctrirelay means comprises cal signal of predeterminedlevel representative of a first relay operable in response to said thirdelectrione limit of said measurement range and generatcal signal, anding a second electrical signal the level of which is a second relayresponsive to the operation of said indicative of whether the level ofsaid first electrifirst relay for actuating said relay switches in saidcal signal is above or below saidsignal of predetervariable gain andzeroing means to connect difmined level, ferent ones of said variableresistors of said variacircuit means responsive to said secondelectrical ble gain and zeroing means into the amplifier signal forgenerating a third electrical signal I 5 means circuit. directly relatedto the level of said second electri- 6. A method for automaticallymeasuring a plurality cal signal, of ranges of differential pressure ofa fluid flowing relay means operable in response to said thirdelecacross a pressure head producing device within a trical signal,pipeline, comprising variable gain means electrically interconnectedwith measuring the differential pressure across the pressaid amplifiermeans for changing the gain of said sure head producing device,amplifier means between a gain appropriate to said generating a firstelectrical signal representative of first measurement range and a gainappropriate to said measured differential pressure within a first asecond measurement range in response to the measurement range, operationof said relay means, and detecting changes in the signal level of saidfirst eleczeroing means electrically interconnected with said amplifiermeans for adjusting the electrical zero of said amplifier means whensaid amplifier means switches from said first measurement range to saidsignal representative of said measured differential pressure within afirst measurement range, and a differential amplifier having a firstsignal input of a trical signal,

generating a second electrical signal the level of which is indicativeof whether the level of said first electrical signal is above or below apredetermined second measurement range, said zeroing means signal levelrepresentative of one limit of said meaoperable in response to saidrelay means. surement range, 2. An apparatus as claimed in claim 1,wherein said generating a third electrical signal representative of asignal level detector means comprises change in level of said secondelectrical signal, and a buffer amplifier for receiving said firstelectrical 5 changing the gain and electrical zero of an amplifiergenerating said first electrical signal in response to said thirdelectrical signal. 7. An apparatus for automatically measuring aplupredetermined level and receiving said first electrical signal fromsaid buffer amplifier as a second signal input, said differentialamplifier generating pipeline, compnsrng as an output said secondelectrical signal the level of which is directly related to thealgebraic difference between said first and second signal inputs.

3. An apparatus as claimed in claim 2, wherein said variable gain meanscomprises a plurality of variable resistors one side of each of which iscommonly connected to a source of voltage, the other side of each ofwhich is connected to one of a plurality of relay contacts, a relayswitch cooperating with said contacts and being connected to a gaincontrol circuit of said amplifier means, each of said variable resistorsbeing set to a predetermined resistance for varying the gain of saidamplifier means by a predetermined value when different ones of saidvariable resistors is connected with the amplifier means circuit.

4. An apparatus as claimed in claim 3, wherein said cooperating withsaid contacts and being connected to a zeroing circuit of said amplifiermeans, each of said variable resistors being set to a a differentialpressure transducer for measuring the differential pressure across thepressure head producing device, I

amplifier means cooperating with said transducer for generating a firstelectrical signal representative of said measured differential pressurewithin a first measurement range,

a buffer amplifier for receiving said first electrical signalrepresentative of said measured differential pressure within a firstmeasurement range,

a differential amplifier having a first signal input of a predeterminedlevel and receiving said first electrical signal from said bufferamplifier as a second signal input, said differential amplifiergenerating as an output a second electrical signal the level of which isdirectly related to the algebraic difference between said first andsecond signal inputs,

circuit means responsive to said second electrical zeroing meanscomprises signal for generating a third electrical signal a plurality ofvariable resistors one side of each of directly related to the level ofsaid second electriwhich is commonly connected to a source of voltcalsignal, age, the other side of each of which is connected to relay meansoperable in response to said third elecone of a plurality of relaycontacts, a relay switch trical signal,

a plurality of variable resistors one side of each of which is commonlyconnected to a source of voltage, the other side of each of which isconnected to one of a plurality of relay contacts, a relay switchcooperating with said contacts and being connected to a gain controlcircuit of said amplifier means, each of said variable resistors beingset to a predetermined resistance for varying the gain of said amplifiermeans by a predetermined value when different ones of said variableresistors are connected with the amplifier means circuit, said relayswitch operating in response to the operation of said relay means, and

a plurality of variable resistors one side of each of which is commonlyconnected to a source of voltcal zero of said amplifier means whendifferent ones of said variable resistors are connected with theamplifier means circuit, said relay switch being operablein response tothe operation of said relay means.

' s m s 1- a

1. An apparatus for automatically measuring a plurality of ranges ofdifferential pressure of a fluid flowing across a pressure headproducing device within a pipeline, comprising a differential pressuretransducer for measuring the differential pressure across the pressurehead producing device, amplifier means cooperating with said transducerfor generating a first electrical signal representative of said measureddifferential pressure within a first measurement range, signal leveldetector means for comparing the signal level of said first electricalsignal against an electrical signal of predetermined levelrepresentative of one limit of said measurement range and generating asecond electrical signal the level of which is indicative of whether thelevel of said first electrical signal is above or below said signal ofpredetermined level, circuit means responsive to said second electricalsignal for generating a third electrical signal directly related to thelevel of said second electrical signal, relay means operable in responseto said third electrical signal, variable gain means electricallyinterconnected with said amplifier means for changing the gain of saidamplifier means between a gain appropriate to said first measurementrange and a gain appropriate to a second measurement range in responseto the operation of said relay means, and zeroing means electricallyinterconnected with said amplifier means for adjusting the electricalzero of said amplifier means when said amplifier means switches fromsaid first measurement range to said second measurement range, saidzeroing means operable in response to said relay means.
 1. An apparatusfor automatically measuring a plurality of ranges of differentialpressure of a fluid flowing across a pressure head producing devicewithin a pipeline, comprising a differential pressure transducer formeasuring the differential pressure across the pressure head producingdevice, amplifier means cooperating with said transducer for generatinga first electrical signal representative of said measured differentialpressure within a first measurement range, signal level detector meansfor comparing the signal level of said first electrical signal againstan electrical signal of predetermined level representative of one limitof said measurement range and generating a second electrical signal thelevel of which is indicative of whether the level of said firstelectrical signal is above or below said signal of predetermined level,circuit means responsive to said second electrical signal for generatinga third electrical signal directly related to the level of said secondelectrical signal, relay means operable in response to said thirdelectrical signal, variable gain means electrically interconnected withsaid amplifier means for changing the gain of said amplifier meansbetween a gain appropriate to said first measurement range and a gainappropriate to a second measurement range in response to the operationof said relay means, and zeroing means electrically interconnected withsaid amplifier means for adjusting the electrical zero of said amplifiermeans when said amplifier means switches from said first measurementrange to said second measurement range, said zeroing means operable inresponse to said relay means.
 2. An apparatus as claimed in claim 1,wherein said signal level detector means comprises a buffer amplifierfor receiving said first electrical signal representative of saidmeasured differential pressure within a first measurement range, and adifferential amplifier having a first signal input of a predeterminedlevel and receiving said first electrical signal from said bufferamplifier as a second signal input, said differential amplifiergenerating as an output said second electrical signal the level of whichis directly related to the algebraic difference between said first andsecond signal inputs.
 3. An apparatus as claimed in claim 2, whereinsaid variAble gain means comprises a plurality of variable resistors oneside of each of which is commonly connected to a source of voltage, theother side of each of which is connected to one of a plurality of relaycontacts, a relay switch cooperating with said contacts and beingconnected to a gain control circuit of said amplifier means, each ofsaid variable resistors being set to a predetermined resistance forvarying the gain of said amplifier means by a predetermined value whendifferent ones of said variable resistors is connected with theamplifier means circuit.
 4. An apparatus as claimed in claim 3, whereinsaid zeroing means comprises a plurality of variable resistors one sideof each of which is commonly connected to a source of voltage, the otherside of each of which is connected to one of a plurality of relaycontacts, a relay switch cooperating with said contacts and beingconnected to a zeroing circuit of said amplifier means, each of saidvariable resistors being set to a predetermined resistance for changingthe electrical zero of said amplifier means when different ones of saidvariable resistors is connected with the amplifier means circuit.
 5. Theapparatus as claimed in claim 4, wherein said relay means comprises afirst relay operable in response to said third electrical signal, and asecond relay responsive to the operation of said first relay foractuating said relay switches in said variable gain and zeroing means toconnect different ones of said variable resistors of said variable gainand zeroing means into the amplifier means circuit.
 6. A method forautomatically measuring a plurality of ranges of differential pressureof a fluid flowing across a pressure head producing device within apipeline, comprising measuring the differential pressure across thepressure head producing device, generating a first electrical signalrepresentative of said measured differential pressure within a firstmeasurement range, detecting changes in the signal level of said firstelectrical signal, generating a second electrical signal the level ofwhich is indicative of whether the level of said first electrical signalis above or below a predetermined signal level representative of onelimit of said measurement range, generating a third electrical signalrepresentative of a change in level of said second electrical signal,and changing the gain and electrical zero of an amplifier generatingsaid first electrical signal in response to said third electricalsignal.